Aluminum alloys possess an excellent combination of mechanical and physical properties. By combining these properties of aluminum alloys with the relatively low density of such alloys, designers are able to produce reliable, lightweight structures. Moreover, a wide range of alloy systems and tempers offer structural designers several options to utilize the appropriate alloys that are specifically designed for particular operating loads or environments.
It is typical for all aluminum alloys to contain grain refining elements such as Zr, Ti, Cr, Mn and V. Grain refining elements help nucleate grains during casting by forming intermetallic phases with Al. For example, Ti will form the TiAl.sub.3 phase which nucleates an .alpha.--aluminum particle as solidification of the molten metal occurs. The large number of TiAl.sub.3 particles help to nucleate .alpha.--aluminum in several areas. Accordingly, the solidified grain structure is much finer than would be observed in an aluminum alloy without grain refining additions, thereby improving the fabricability during subsequent hot working operations.
Another function of a grain refining element is to form coherent (e.g., Al.sub.3 Zr) and non-coherent (e.g., Al.sub.12 Mg.sub.2 Cr and Al.sub.20 Mn.sub.3 Cu.sub.2) insoluble phases during casting and ingot preheat. These thermally stable dispersoids prevent or delay static recrystallization during processing. In addition, the dispersoid phase pins the elongated grain boundaries that develop during processing and inhibits recrystallization that would otherwise occur during the solution heat treatment steps.
Among all alloying elements used to strengthen wrought aluminum alloys, scandium (Sc), despite its rare occurrence, has received significant attention. For instance, U.S. Pat. No. 3,619,181 to Willey discloses the addition of Sc to a wide range of binary, ternary and multicomponent alloy systems. It is claimed that the aluminum alloys that can be strengthened with Sc additions include wrought aluminum alloys identified by the Aluminum Association such as 7075, 7079, 7178, 7005, 7039, 6061, 6351, 6161, 6063, 5005, 5050, 5052, 5083, 5454, 5456, 3003, 3005, 2014, 2017, 2618, 2219, 2020 and 2024. Several model alloy systems were fabricated with and without Sc additions and tested for strength and ductility. Additions of 0.2 to 0.4 weight percent Sc caused both tensile strength and yield strength to increase by between 6 and 50 percent. The use of a cold working step for the Sc alloys caused further increases in strength.
Sawtell and Jensen reported enhanced strength and superplastic formability when adding Sc to the wrought Al-Mg system (see "Mechanical Properties and Microstructures of Al-Mg-Sc alloys," Sawtell, R. R. and Jensen C. L., Metallurgical Transactions, V. 21A, February, 1990, pp. 421-430). It was stated that the equilibrium precipitate phase Al.sub.3 Sc is the most potent strengthener known in the aluminum based alloy system on an equal atomic fraction basis.
U.S. Pat. No. 5,055,257 to Chakrabarti et al. documents the enhancement of superplastic forming by using the thermal stability of the Al.sub.3 Sc precipitates. Improvement in total superplastic elongation was achieved in a wrought Al-Mg alloy. It was also noted that the total time to achieve a certain strain level was two orders of magnitude greater than previously achieved with other superplastic alloys. Based on this information, it was emphasized that similar mechanistic improvements can be realized for other wrought aluminum alloys in the 2XXX and 7XXX systems.
U.S. Patent application, from which this patent application is a continuation-in-part thereof, Ser. No. 08/249,023, filed May 25, 1994, discloses the use of Sc in combination with several other dispersoid forming elements to enhance the weldability and weld strength of aluminum alloys in the 2XXX, 5XXX, 6XXX and 7XXX wrought alloy systems. The Sc additions are especially advantageous when added to both the base alloy to be welded and the filler alloy. An alloy design technique was used whereby conventional grain refining elements such as Cr and Mn were replaced by Sc+Zr. In one particularly interesting example, alloy 6061 was subjected to a weldability test known as the "patch test" to assess its resistance to hot cracking. The total crack length measurements ranged from 31.8 mm to 43.4 mm, corroborating published data that show 6061 as the most crack sensitive alloy among all aluminum alloys. When the Cr was removed and replaced by Sc and Zr, cracking during the patch test was reduced to 0 mm. Thus, the approach to replace conventional grain refining elements with Sc+Zr can convert the worst known alloy with regard to hot cracking resistance to one that displays no hot cracking.
U.S. patent application Ser. No. 08/311,958, filed Sep. 26, 1994, discloses the use of Sc to greatly improve the strength of aluminum casting alloys. A 356 type alloy, which usually displays a 43% lower yield strength value relative to 357, was alloyed with Sc to produce a 33% strength advantage relative to 357 as measured by bend testing. Accordingly, several other aluminum casting alloys were proposed for property improvement by using the principles disclosed in the invention.
Alloy development efforts can, of course, concentrate on any number of desired objectives for a given product application. Two common design objectives for some alloy systems are enhanced strength characteristics and reduced weight. One product area where high mechanical properties and light weight is becoming paramount to performance is the field of athletic equipment. A specific athletic endeavor where use of advanced materials is increasingly evident is the bicycle, and in particular, mountain bicycles that are designed for demanding off road use such as mountain trails. The high performance models are usually comprised of a welded aluminum or titanium frame with several components such as handle bars, pedals, seatposts, wheel rims, crank arms, suspension forks, etc. that are designed using light weight, high strength metal alloys.
The importance of weight reduction in bicycles is evidenced by the significant growth of the after-market for bicycle components. Advertisements for such parts usually specify the weight of the component in grams so the rider can determine whether replacement of an existing part can be made to reduce the overall weight of the bicycle structure. This approach can be taken in lieu of purchasing an entire bicycle.
In a recent article ("How to Shave Weight", Mountain Bike Action, December 1994, p. 78), the importance of decreasing the weight of a bicycle was emphasized, and several steps were divulged to enable riders to decrease the total weight of the bicycle by replacing several components. As an example, a 135 gram (g) titanium handlebar or a 148 g aluminum alloy handlebar can replace an existing steel alloy handlebar, thereby shaving 100 to 200 g. The same principle was applied to seatposts, saddles, wheels and several other parts. When added together, it was stated that several pounds of weight can be shaved. This weight reduction significantly improves the climbing ability of the rider without sacrificing the structural integrity of the bicycle.
Aerospace structures are constructed primarily from aluminum alloys. Since designers are continually seeking alloys with enhanced properties to decrease the weight of aircraft, aluminum companies devote a significant amount of research and development resources to introduce new aluminum alloys with enhanced properties. Because the aerospace structure production infrastructure is already established for aluminum alloys, the typical design approach is to introduce a new alloy with improved properties that can be integrated into the structure using conventional manufacturing methods.
In simplistic terms, an alloy with improved strength can be introduced with a thickness reduction that is proportional to the strength advantage. By using a space launch vehicle as an example, it is evident that a new alloy with a 10% strength advantage can be used to decrease the thickness of the propellant tank wall by 10% while maintaining an equivalent load carrying capacity of the original alloy. On a structure such as the Space Shuttle's External Tank, a 10% reduction throughout the 66,000 pound tank structure would make a significant impact by shaving 6600 pounds. It should be noted that other properties such as stress corrosion resistance and fracture toughness, along with manufacturing processes such as welding and forming, must be thoroughly addressed before introducing a new alloy.
It is estimated that a one pound reduction in airplane structural weight will save 300 to 400 gallons of fuel over the projected lifetime of the aircraft (see Quist, W. E. et.al., "Aluminum-Lithium Alloys for Aircraft Structure--An Overview," Aluminum-Lithium Alloys II Conference Proceedings, Starke, E. A. Jr. and Sanders, T. H. Jr., eds., 1983, pp. 313-334.). With the potential for saving hundreds or thousands of pounds by replacement of existing aluminum alloys with alloys that display incremental property improvements, it is evident that airplane fuel consumption can be significantly reduced.
Recent government mandates have been issued to automotive manufacturers to improve fuel efficiency of vehicles and thereby decrease emissions that are harmful to the environment. Accordingly, the design strategy to reduce the vehicle weight by using aluminum alloys in place of steel is gaining momentum in the automotive industry. Automotive designers, however, must maintain the crashworthiness of the vehicle at an acceptable level while achieving weight reduction.
The benefits of vehicular weight reduction apply not only to consumer passenger vehicles, but other types as well. For instance, major transport organizations such as an urban-based bus systems could greatly benefit from reduced vehicular weights and realize a significant reduction of fuel consumption and air pollution in a specific geographical area. Moreover, a truck fleet which transports liquid or cryogenic liquid products can not only benefit from weight reduction for the above-noted reasons, but also by reducing trucking fees that are based on the total weight of the truck. Accordingly, the fee amount can be saved for every trip that a truck makes throughout the life of the vehicle.
A fourth product area where reduced weight would be advantageous is marine structures. By employing the aforementioned principles utilized in aerospace and automotive structures, marine structures can be improved by introducing high strength, corrosion resistant alloys.